Underwater vessels such as submarines are well known. More recently, mini-submarines have been used to monitor fish stocks for farm cultured salmon and for inspecting submerged structures in deep-sea oil exploration. Monitoring and inspection is often performed using optical cameras for capturing images of underwater regions, as well as employing active sonar equipment for emitting sonic energy and receiving corresponding sonic reflections from submerged structures.
However, such methods of monitoring and inspection do not enable certain types of defects in structures to be detected, especially those of an electrical nature. More recently, there has been increased use of underwater cables for coupling power between electrical networks separated by regions of water, for example between southern Sweden and the Danish island of Sjælland, as well as across the English Channel between England and France. Moreover, underwater electrical cables are employed to provide power to submerged equipment during oil and gas exploration and extraction. Furthermore, telecommunications links including optical fibre waveguides often include electrical supply lines for providing power to erbium doped fibre optical amplifiers (EDFAs) for providing periodic regeneration of optical signals to compensate for optical attenuation occurring along the optical fibre waveguides.
Raising an underwater cable for repair is often an extremely expensive operation and there is a great need to be able to detect locations whereat submerged cables and electrical equipment are potentially defective or have developed faults. Optical inspection often provides an inadequate approach for identifying and localizing defects and faults.
In U.S. Pat. No. 7,116,108, which is based on published international PCT patent application no. WO 03/104844 (PCT/US03/18522), there is described a system for mapping electrical conductivity of the seafloor. The system incorporates several data logging units. Each unit is an assembly adapted for being deployed at a location on the seafloor for measuring horizontal electric and magnetic fields there. A vertically-orientated substantially rigid arm extends vertically from the unit assembly and includes a pair of vertically-displaced electrodes disposed on the arm to create a vertically-orientated dipole antenna. The electrodes of the arm are in electrical communication with an amplifier located within the assembly which generates an amplified signal which is then provided to a data logging processor also located within the assembly. The processor collects time series of amplified electric field and magnetic signals over a predetermined period of time.
Moreover, in a published U.S. Pat. No. 6,867,596, there is disclosed a method of detecting breakdowns in insulation and corresponding earth faults in a buried land cable; a test signal is applied to the cable, the signal being detected using a differential voltage probe placed in a conductive medium in near proximity to the cable. The differential voltage probe generates a received signal which is processed in a signal processing circuit operable to decompose components of the received signal corresponding to the test signal. The decomposed components are then subject to a phase comparison to determine a direction of current leakage associated with the earth fault.
Hostile aquatic environments can be subject to considerable water flows and other disturbances which render techniques hitherto employed unsuitable. There thus arises a challenging technical problem of detecting electrical faults in hostile aquatic environments.
A standard manner of testing for earth-connection faults in power distribution networks is to perform an insulation tests by “megging”. When implementing “megging”, a high potential signal is applied to a conductor surrounded by an insulator and a resistance of the insulator is measured in response to the signal being applied. Performing tests by “megging” is not feasible in a situation for detecting an earth fault on an electrically-floating secondary winding of an underwater transformer. There is no known contemporary method that is able to detect an earth fault associated with such a secondary winding of an underwater transformer without special instrumentation being built into the transformer for detecting occurrence of any such faults. This special instrumentation is susceptible to increasing installation complexity and cost and represents a sub-optimal solution. Transmission of a test signal into a transformer circuit is not satisfactory when the secondary side of the transformer is positioned in the sea bed and in operation.